8 - Cell Polarity Flashcards
What is Cell Polarity?
the organisation of proteins inside + at the surface of cells
regions have distinct protein composition
the cell can thereby have diff capabilities, morphologies and functions
Why is cell polarity necessary for?
the cell to be able to generate a wide variety of forms to perform a diverse array of functions
What are the key functional requirements to be able to polarise a cell?
- Marking the site
- Decoding the site - cell must know something has happened
- Establishing the site - key proteins go to site - organise cytoskeleton, make other prot etc
- maintaining the site - feedback loops
(5. How to unpolarise)
Why is budding yeast (Saccharomyces cerevisiae) good for studying cell polarity?
- yeast undergoes sig morphological changes in response to external + internal sig
- is genetically tractable - entire genome is known + annotated
- has been used to understand fundamental aspects of key cell processes inc cell cycle, secretion + cell polarity
Internal changes of budding yeast:
in response to growth + division signals
e.g. growth of bud + cytokinesis
External changes of budding yeast:
in response to pheromones (for mating)
and nutritional signals (cells can elongate)
What does budding yeast need to grow and divide?
Cell polarity
How do yeast cells bud?
bud + divide in precise spatial patterns
can be followed by staining w/ fluorescent dye - calcifluor
binds to chitin
allows birth scars (mark sites of previous sites of separation) - to be viewed as bright rings on the cell wall
What does the position of the new cell bud depend on?
if the cell is haploid or diploid
Where do new haploid cell buds form?
haploid a and alpha cells bud in axial pattern
- both mother + daughter cells are constrained to form buds immediately adjacent to previous site of cell separation
Where do new diploid cell buds form?
bud in bipolar manner
mother + daughter constrained to form bud at the poles of ellipsoidal cells
allows to explore wider environment
What genes are required for the yeast axial budding pattern?
BUD10, BUD3, BUD4 and the septins
What do products of BUD10, BUD3, BUD4 and the septins do?
are involved in marking the mother bud neck during one cycle as a site for budding in the next cycle
mutations do not affect haploid cells
Haploid cells now mostly bud in bipolar manner
What genes are required for the yeast bipolar budding pattern?
BUD8,BUD9,RAX2 and components of actin skeleton are involved
What do products of BUD8,BUD9,RAX2 and components of actin skeleton are involved
do?
They mark the ends of the diploid cells
Haploid mutants in these genes still use the axial pattern but the bipolar pattern is disrupted
What genes are required for both axial and bipolar budding patterning?
BUD1, BUD2,BUD5
What do proteins encoded by BUD1, BUD2, BUD5 do?
decode the axial + bipolar marks + signal to the machinery involved in generating the polarity axis
Mutations in these genes cause a random budding pattern in both haploid + diploid cells
How is the site decoded?
BUD1, BUD2, BUD5 function together to signal to the polarity establishment machinery the position of the bud site cortical landmarks
function together in a GTPase cycle
How is the site established?
cell integrates spacial cues from budding landmarks
information fed to polarity establishment machinery
responsible for cell cytoskeleton + other cell components
What are the most important proteins involved in polarity establishment?
the family of rho-GTPases
What are the most important proteins involved in polarity establishment in yeast?
Cdc42
What mutations in yeast stop the formation of new buds?
Genes - cdc24, cdc43, cdc42
How is Cdc42 temperature sensitive?
is a temperature sensitive mutant
At 24C cells can polarise, form a bud, grow + divide
At 37C cells show isotropic growth (grow all over surface) + cannot establish an axis of polarity
How is Cdc42 regulated?
through cycles of activation + inactivation by its binding partners Cdc24 (a GEF) and several GAPs
How does Cdc42 function to establish cell polarity?
The GEF for Cdc42 (Cdc24) binds to the active form of BUD1 at sites marked for budding
Cdc24 then binds BUD1 and can then activate Cdc42 to allow the polarity site to become established
Can recruit a number of proteins
Mating in budding cell yeast and polarisation
Haploid yeast cells can polarise and redirect growth to facilitate mating
response is chemotrophic due to mating pheromones being secreted from different cell types
Explain MATa and MAT alpha cells?
Involved in mating in budding yeast
MATa cells secrete a-factor
MATalpha cells secrete alpha-factor
MATa cells have a receptor on their cell surface (Ste2)
- can bind to alpha factor
MATalpha cells have a receptor (Ste3) - can bind to a factor
What receptors detect + bind to pheromones?
members of conserved g-coupled receptor family
They interact w/ a heterotrimeric g-protein that orchestrated the downstream cell response
Cell cycle arrest
When do budding yeast cells mate?
Only mate during G1
Far1 binds to heterotrimeric g protein***
Sec3 - more elongated growth
what key proteins require asymmetric inheritance in daughter cells?
key proteins (including myosins Myo2 and Myo4)required asymmetric inheritance of specific factors (proteins and mRNAs)
what does the degradation of cargo adaptors in daughter cells do?
prevents backwards movement of organelles
What is the fungus candida albicans?
normally a beign member of mucosal flora
50% of people carry it
However it commonly causes mucosal disease
It becomes invasive there is substantial morbidity
- in vulnerable patients causes life threatening bloodstream infections
Freq seen in AIDS patients, premature babies and terminally ill patients
The fungus can leave the bloodstream and penetrate cell tissues and organs
Explain how candida albicans is dimorphic?
able to switch btwn yeast and hyphal forms
can grow in both forms
- this is central to the virulence
Hyphal forms of candida albicans:
Hyphal formation - stimulated at 37C by serum or neutral pH
Hyphae are more adherent to mammalian cells + imp for tissue penetration
Yeast forms of candida albicans:
Yeast cells taken up my macrophages - can switch to filamentous growth + lyse the macrophage
Yeast cells - carried more effectively in the bloodstream promoting fungal dissemination in the body
What is cell polarity crucial for?
- Asymmetric cell division - including cell fate decisions
- Epithelial cells - to make an effective barrier
3.Cell migration e.g. In development
Whitman (1878)
studied leeches and showed that distinct cytoplasmic domains are differently partitioned to descendants and that these differences were reflected in different cell lineages
Conklin (1905)
identified 5 different cytoplasm types in the ascidina oocyte that were differently inherited to determine tissue types
What are the two main routes of diversity in daughter cells?
- Polar mother cells could divide to generate daughters that have inherited different components
- Daughters could be equal at birth but then become different by exposure to different environmental signals
How does asymmetric cell division take place?
- Establishment of an axis of polarity (this involves marking a site, signalling and establishing - as discussed earlier)
- Mitotic spindle is positioned along the axis
- Differential distribution of cell fate determinants to daughter cells
What do PAR proteins do?
PAR proteins form the core of a cell polarity network in many animal cells and in many developmental contexts
When does polarisation start?
Polarisation starts w/ entry of sperm to oocyte
Position of sperm entry defines the posterior end of the zygote
How does the zygote divide?
The zygote - also called PO cell - divides asymmetrically along A-P axis
Produces a larger anterior cells (AB) and a smaller posterior cell (P1)
The daughters are different in size and are committed to diff cell fates
When is symmetry broken in the oocyte?
at fertilization
the sperm delivers a microtubule organising centre (MTOC)
This site becomes posterior pole and so defined the axis of polarity
What does the recruitment of PAR1 and PAR2 by microtubules in the oocyte lead to?
antagonises anterior Par proteins and they accumulate at anterior cortical domain
- this results in distinct localizations of the par proteins
Where do different PAR proteins localise?
Par3.Par6/aPkc localise to the anterior cortex
Par1 and Par2 are at the posterior cortex and Par maintains the boundary
where are neuroblasts found in drosophila?
found within a specific region of an epithelial monolayer
- called the ventral neuroectoderm
Drosophila asymmetric division:
polarity established when cell is still in neuroectoderm layer
neuroblasts delaminate Par3 Bazooka/Par6 are found in a stalk that continues to extend into the epithelium
After delamination, they continue to localise to the apical region
Baz anchors another complex (insc/Pins) at the membrane in order to orient the mitotic spindle
Scribble complex helps in spindle alignment
what cell fate determinants are found in drosophila
transported in the basal direction to the ganglion mother cell
includes factors such as prospero and staufen - regulate expression of specific genes in the GMC
Following cell division the GMC has a different fate because of asymmetric inheritance of these determinants
What are the 3 main activities required for movement
- Protrusion - the pushing out of the plasma membrane in front of the cell
- Attachment - the actin cytoskeleton inside the cell is attached via interacting proteins across the plasma membrane to the substratum (e.g. Extracellular matrix)
- Traction - the bulk of the cell body is drawn forward through the process of contraction
What role do actin structures play in migration?
diff types of protrusion
- including filopodia + lamellipodia
Are filled w/ filaments of the cytoskeleton protein -actin
Filopodia or microspikes are a dense core of bundled actin filaments
- are involved in the contractility required to move body of cell forward
What Rho GTPases are involved in cell migration?
Cdc42, Rac and Rho
diff GTPases responsible for generating different actin structures in cells
What is chemotaxis?
This is the movement of cells towards/ away from a signal
- e.g. A diffusible chemical
An example is the movement of a neutrophil moving towards a site of bacterial infection
